A variety of medical devices exist that are implanted inside the body and undergo a dimensional change. For example, a bone lengthening device is one type of implantable device that is typically inserted into first and second portions of a severed or broken bone. The device is then periodically lengthened to distract or grow the bone over a period of time. Such adjustments made to the bone lengthening device may be invasive or even non-invasive. As another example, growing rods or distraction devices may be secured to a subject's spine. These devices may be used to correct a medical condition such as scoliosis. In still other applications, these devices may be used to increase the distance between adjacent vertebrae to reduce symptoms associated with lumbar spinal stenosis or pinched nerves. Other bones such as the jaw bone may include an implantable medical device that is configured to elongate over time.
Regardless of the nature in which the implanted medical device is used, there often is a need to determine the absolute length of the implant as it exists inside the patient at any given moment. As an example, after the implanted medical device has undergone a length adjustment there is a need to determine whether or not the desired quantity of lengthening was indeed achieved. In addition, devices may change dimensions after the adjustment has been made (whether manual or non-invasive). For example, normal physiological movement of the subject may cause additional shortening or lengthening of the device after adjustment of the device. In these instances, it would be beneficial to know the actual length of the device in between adjustment procedures. For example, a physician might want to know if the device has strayed too far in either direction to warrant an additional adjustment.
U.S. Patent Application Publication No. 2010/0094302 discloses a non-invasive medical implant device that uses microphone sensor on an external adjustment device to sense when an internally-located magnet is undergoing rotation. Specifically, the microphone sensor picks up an acoustic signal (e.g., click) that is periodically generated by rotation of an internal magnet that is part of the implantable medical device. By counting the number of clicks, the external adjustment device can then translate this into an estimated length of the device. While such a method does provide a means to determine the length of the implanted medical device there is the possibility that one or more of the click signals may not be detected by the external adjustment device. In this instance, the actual length of the implanted medical device may then be different from the length that is calculated or otherwise determined by the external adjustment device. Further, while the external adjustment device may store the most current length of the device as determined by the sensed signals, it is possible that the subject may return to a different physician for his or her next adjustment procedure. Unless the size of the implanted medical device is stored locally on or with the patient (e.g., RFID or a card carried by the patient), the next physician will not know the most recent sizing of the device. Moreover, as stated above, there is the possibility that the implanted device may change lengths in between adjustment procedures. There thus is a need for methods and devices that will determine the absolute length of an implantable medical device at any given movement.